The E. coli ffh gene is necessary for viability and efficient protein export

Homologues of the gene encoding the 54K (M_r 54,000) subunit of the mammalian signal recognition particle have been identified in different organisms^1-5. The Escherichia coli homologue, termed ffh (for fifty-four homologue), specifies a protein (Ffh) that shares many properties with its eukaryotic counterpart, including association with mammalian 7S RNA⁶ and the ability to bind signal sequences specifically ^7,8. Ffh also associates with E. coli 4.5S RNA, showing that it can form a ribonucleoprotein complex in prokaryotes^6,9,10. These results are intriguing because extensive genetic and biochemical characterization of E. coli failed to identify a signal recognition particle-like mechanism for protein export¹¹. Here we address this issue directly by construction of a strain in which ffh expression is arabinose-dependent. Results of depletion experiments indicate that Ffh is important in protein translocation.